Modern jet engine systems comprise gas turbine engines that run on jet fuel. Under normal operating conditions, jet fuel is heated by the hot components or regions of the gas turbine engines, which include the fuel nozzles, fuel nozzle support assemblies, and heat exchangers. Modern jet engine systems use the jet fuel's heat sink capability for cooling various aircraft systems, including hydraulic, electronic, and lubrication systems. However, heat management and, ultimately, performance of the jet engine system and airframe is a delicate balance between (i) running fuel systems cooler and incurring performance, cost, and weight penalties by use of air cooling, or (ii) running systems as hot as possible and causing problems associated with unacceptable deposition rates. Accordingly, engineers often design jet engine systems to take maximum advantage of the thermal stability of currently available fuels.
Trends in higher whole engine system performance as well as airframe and engine heat loads, coupled with simultaneous reductions in fuel consumption, are forcing fuel system temperatures to increase even further. Therefore, many modern high performance jet engine systems utilize thermally stressed fuels. At high temperatures, however, less stable species in the thermally stressed jet fuel may undergo oxidation reactions that produce gums, lacquers, particulates, and coke deposits. These resultants may cause a number of problems, including blockage of filters, loss of heat exchanger efficiency, stiction or hysteresis of sliding components in control units, and fouling of injectors and distortion of spray patterns. For example, oxidation of thermally stressed jet fuel may result in deposits or particulate that blocks engine fuel nozzles, thereby causing damage to the engine hot sections due to distorted fuel spray patterns, especially the combustor region. Accordingly, a jet fuel's thermal stability is critical to achieving optimum performance of modern gas turbine engines.
The current standard for evaluating a jet fuel's thermal oxidation is the Standard Test Method for Thermal Stability of Aviation Turbine Fuels, designation D3241, IP323, as published by the American Society for Testing and Materials International (“ASTM International”). This test method mimics the thermal stress conditions encountered by jet fuel in operation and, despite being developed in the early 1970s, remains the best method to evaluate jet fuel thermal stability. More specifically, the D3241 test method sets forth a procedure for rating the tendency of jet fuels to deposit decomposition products within a fuel system. The D3241 test method is performed in two (2) phases. The first phase mimics the fuel conditions present during airplane engine operation, whereas the second phase quantifies the oxidation thermal deposits formed during the first phase.
Various laboratory devices, known as rigs, have been developed since that time to facilitate the D3241 test method. These rigs subject an aluminum heater tube to sample jet fuel under conditions mimicking those encountered during actual engine operation. However, these rigs are difficult to use and require substantial expertise when installing the heater tube within the test section and when preparing the jet fuel sample. Moreover, these known rigs include pump systems that move the fuel sample through the test section, but often have leaks, inconsistent flow rates, and micro-ruptures, and are expensive to operate and maintain. Furthermore, these known rigs have primitive temperature control systems that impact the test results and reproducibility of the same.